The present invention generally relates to driving circuits for D.C. commutatorless motors, and more particularly to a driving circuit for a D.C. commutatorless motor, which is designed so that a current does not unnecessarily flow through power switching elements even when the amplitude of an output signal of a rotor position detecting means is set to a small amplitude in order to prevent rapid switching of currents with respect to stator coils, the utilization efficiency of the stator coils is improved, and the number of power switching elements which are required is reduced.
As a conventional driving circuit for a D.C. commutatorless motor, there was a driving circuit designed to flow currents in only one direction with respect to four stator coils. However, this conventional driving circuit had a deficiency in that the torque obtained by driving the D.C. commutatorless motor by such a conventional driving circuit was small. Hence, as will be described hereinafter in conjunction with the drawings, there was another conventional driving circuit designed to flow currents in both forward and reverse directions with respect to the stator coils. According to this other conventional driving circuit, the utilization efficiency of the stator coils is improved compared to the former conventional driving circuit. Moreover, there was an advantage in that a large torque can be obtained by driving the D.C. commutatorless motor by the latter conventional driving circuit. On the other hand, however, the latter conventional driving circuit required eight power transistors through which large currents flow, for example, and the manufacturing cost of the driving circuit was high. Furthermore, the latter conventional driving circuit was disadvantageous in that the size of the motor became large, and a large number of processes were required to assemble the motor.
In addition, when output voltages of Hall elements are sufficiently large, the currents flowing through the stator coils are rapidly switched. However, when such switching of the currents is rapid, the rotor will not rotate smoothly. Especially when the frequency of the current switching is near the resonance point of mechanical vibration in the rotor system (rotor shaft, magnets, and the like), such phenomenon in which the rotor will not rotate smoothly becomes particularly notable. The output voltages of the Hall elements may be set to small voltages, however, in this case, the current switching with respect to the stator coils cannot be carried out in a satisfactory manner. Especially in a D.C. commutatorless motor comprising Hall elements which use indium antimonide (InSb), the Hall elements have relatively high negative temperature coefficients in relation to the output voltages thereof. Thus, in such a D.C. commutatorless motor comprising the Hall elements which use indium antimonide, the output voltages of the Hall elements decrease to the extreme degree under high ambient temperature conditions, and effects the current switching with respect to the stator coils. In this case, an interval will exist in which collector currents of switching transistors flow simultaneously, and the power source current will become large. Thus, in extreme cases, the power transistors may be destroyed, and even if the power transistors do not become destroyed, the power source efficiency of the motor will be low.